FoxO3a‐Mediated Modulation of PD‐L1 Expression and Inhibition by Dihydroartemisinin in Triple‐Negative Breast Cancer

ABSTRACT Tumour immunotherapy targeting PD‐1/PD‐L1 shows promise, but the regulatory mechanisms of PD‐L1 and its small‐molecule modulators remain unclear. This study investigated FoxO3a‐mediated PD‐L1 regulation and the PD‐L1‐inhibitory role of dihydroartemisinin (DA) in triple‐negative breast cancer (TNBC). FoxO3a overexpression significantly increased PD‐L1 expression and impaired T cell‐mediated cytotoxicity, while knockdown exerted opposite effects in TNBC cells. Promoter motif analysis and dual‐luciferase assays revealed FoxO3a binding to the s155 site on the PD‐L1 promoter in MDA‐MB‐231 cells; mutation of s155 abolished this interaction. ChIP‐PCR confirmed FoxO3a binding to the PD‐L1 promoter at s155. Furthermore, DA, a clinical antimalarial, reduced PD‐L1 and FoxO3a levels, sensitising TNBC cells to T cell killing in TNBC cells. Mechanistically, DA enhanced IRE1/IKK phosphorylation, promoting FoxO3a Ser644 phosphorylation and ubiquitination. Crucially, s155 was required for DA‐induced PD‐L1 suppression in MDA‐MB‐231 cells. These findings demonstrate PD‐L1 as a direct transcriptional target of FoxO3a and identify DA as a potential TNBC therapeutic targeting the IRE1/IKK/FoxO3a/PD‐L1 axis.